In lithographic printing, a presensitized plate comprising a photosensitive body of a diazo compound or the like and an aluminum alloy plate is subjected to processing such as image exposure, developing and the like to obtain a plate on which an image portion has been formed, and is wrapped around the cylindrical plate drum of a press, and under the presence of moisturizing water adhered to the non-image portions, ink is adhered to the image portions, and this adhered ink is transferred to a rubber blanket, and printed on the surface of a paper.
As the support body of the above presensitized plate, it is usual to use an aluminum alloy plate which has been subjected to a surface treatment such as a roughening process by electrolytic etching (graining), an anodic oxidation treatment or the like. As an aluminum alloy used for this type of application, JIS 1050 (pure Al, having a purity 99.5% and above), JIS 1100 (an alloy of Al and 0.05 to 0.20% Cu) and JIS 3003 (an alloy of Al, 0.05 to 0.20% Cu, and 1.5% Mn) were mainly used at first.
Various characteristics are required for this type of aluminum alloy plate for use as a lithographic printing plate, such as:
(1) A surface uniformly roughened by electrolytic etching.
(2) Good adhesiveness for a photosensitizer.
(3) Contamination does not arise at the image portion during printing.
However, because the products specified by JIS 1050, JIS 1100, and JIS 3003 cannot sufficiently satisfy all of the above requirements, many improvements have been made to the alloy compositions and the state of the surface that can be obtained.
For example, the surface roughening process is carried out to fix and adhere the photosensitive layer, in addition to providing water retentivity to the surface of the aluminum alloy plate. However, with prior surface roughening treatments, unetched portions would arise on the roughened surface, and the distribution of the pits formed by the surface roughening was uneven, which had considerably adverse effect on the performance of the printing plate, and there has been demand to improve the surface state.
In the past, from the above viewpoint, improvements in materials have been tested, and as one method, it has been proposed to add special elements to the material. For example, Japanese Unexamined Patent Application, First Publication No. Hei 11-115333 discloses a method wherein by the addition of a predetermined amount of Ni, the formation of pits is promoted, and the etchability is increased; Japanese Unexamined Patent Application, First Publication No. Sho 58-210144 discloses a method wherein by the addition of Sn, In and Ga, micropits are formed and the etchability is increased.
However, even if the above special elements are added, the above mentioned requirements cannot be met, and further, by adding these particular elements, there are the problems that increases in material costs are incurred, and the recyclability is impaired.
Further, a method has been proposed for improving the etchability without adding special elements, by focusing on and controlling the size and density of the intermetallic compound (Japanese Unexamined Patent Application, First Publication No. Hei 11-151870). In this method, the intermetallic compound becomes the starting point of the etching, and micropits are uniformly formed. However, the etchability according to this method cannot be sufficiently improved, and the above requirements cannot be satisfied.
Based on the research of the present inventors, it was learned that it is not possible to obtain sufficient etchability by controlling the size and density of the above intermetallic compounds, because the chemical solubility of these intermetallic compounds is unexpectedly large, and they dissolve in the electrolytic solution, and are eliminated, and as a result, they do not sufficiently function as the starting points of etching pits. As a result of further progress in their research, it was found that by moderately dispersing particles of a metastable AlFe type intermetallic compound, the etchability is greatly improved, and the above requirements can be sufficiently met.
Further, the present inventors learned as a result of further progress in research on this type of presensitized plate, that in the case of subjecting an aluminum alloy plate to an electrolytic etching process by immersion in an electrolytic treatment solution, as the aluminum plate is moved from the roll, at right angles to the direction of the aluminum plate (the direction of moving the aluminum plate), streaking, the primary cause of uneven etching, readily occurs on the aluminum plate. This streaking occurs especially readily when the line speed is increased and the electrolytic etching treatment time is short, and at the portions where unevenness arises, i. e., at the portions where the surface roughening is shallow, the streaking remains even in the final state of the product of a presensitized plate provided with a photosensitive layer, and this is connected with an unfavorable external appearance, and there is great concern that the adhesion of the photosensitive film will be low and the durability will be reduced.
Further, for this type of presensitized plate, in some types of products, an even higher degree of strength is demanded. For example, in the case that the presensitized plate is chucked on a roll, a bend is made at the end portion to fix and wrap it around the print cylinder, but this bend is in a direction perpendicular to the direction of rolling, namely it occurs in a direction parallel to the above streaking, and therefore there is the problem that there is concern of cracks arising in the presensitized plate when it is bent.
For example, in order to prevent the occurrence of the above streaking, it can be considered to subject it to a strong electrolytic etching treatment, but the present inventors discovered that as a result of this, the anode site undergoes stronger etching than the cathode site, and therefore, pits which are initially moderately formed have a tendency to become overetched, and these overetched portions can easily overlap the above mentioned bent portions. If the above cracks develop in a presensitized plate, ruptures are likely to form with these cracks as their origin.